Multiple sclerosis (MS) is the most common disabling neurological disease of young adulthood. It is an incurable autoimmune disorder involving oligodendrocyte loss, demyelination, and an inability to remyelinate damaged brain. Remyelination declines with age and MS progression, where oligodendrocyte precursor cells (OPCs) are actively prevented from differentiating into myelinating cells. Current therapies are only directed at reducing demyelination via immune suppression which reduces immune competence. Importantly, no therapies exist to prevent the inevitable decline in remyelination. This project is designed to remove this void. OPC differentiation into myelinating cells is regulated by certain microRNAs (miRNAs) that are reduced in MS lesions. Notably, blood from young rats enhances remyelination of aged brain. The rationale for this work comes from pilot data showing that blood-derived exosomes, small vesicles released from cells, contain miRNAs that trigger remyelination. Non-immunogenic exosomal miRNAs from the serum of young and environmentally enriched (EE) rats enhanced remyelination. The long-term goal of our research is to develop neurotherapeutics based on understanding how EE protects brain. The overall goal of this application is to develop exosomes containing miRNAs as a therapeutic for brain remyelination. The central hypothesis is that these exosomes will enhance remyelination of brain and can be crafted into a therapy for MS. The project has four Aims: AIM 1: Establish interferon-gamma (IFN-?) -stimulated dendritic cells (DCs) as an effective source of exosomes that mimic EE-exosomes to enhance remyelination without injury or immune reaction in slice cultures. AIM 2: Establish an efficient means of loading DC exosomes with fabricated targeted miRNA species and establish their ability to enhance remyelination without injury or immune reaction in slice cultures compared to EE-exosomes and IFN-?-stimulated DC exosomes. AIM 3: Establish the optimal efficacy of intravenously (then nasally) delivered (Aims 1-2) exosomes to promote remyelination without injury or immune reaction in a rat lysolecithin model of MS demyelination. AIM 4: Establish the optimal efficacy of intravenously (then nasally) administered (Aims 1-2) exosomes to promote remyelination in a mouse model of MS [experimental autoimmune encephalomyelitis (EAE)] without injury or enhanced immune reaction. Outcome measures will be: 1) OPC, differentiated oligodendrocyte, and compact myelin levels, targeted miRNAs & target levels; 2) toxicity, measured by cell death, liver and renal function assays, plus oncogenesis; 3) immune activation assayed by serum and brain markers; and 4) clinical measures of disease severity. The work is innovative because it uses naturally occurring signaling molecules and vehicles for delivery of EE-like enhancement of myelination. The proposed research is significant because it will establish the preclinical feasibility and utility of EE-based miRNA-containing exosomes as a novel therapeutic to remyelinate MS- damaged brain without immune compromise.